Make sure to work with the correct command prefix

The previously used variable seemed to always be initialized with
the "/packit" value, which would result in incorrect behaviour
in Fedora CI as the expected value in this case would be
"/packit-ci" instead. Correct value should now be in use.

Author: betulependule

packit_service/worker/jobs.py

@@ -1139,9 +1139,13 @@ def is_help_comment(self, comment: str) -> bool:
         Returns:
             `True`, if is help comment, `False` otherwise.
         """
+        packit_comment_command_prefix = (
+            "/packit-ci" if comment.startswith("/packit-ci") else "/packit"
+        )
+
         command = get_packit_commands_from_comment(
             comment,
-            self.service_config.comment_command_prefix,
+            packit_comment_command_prefix,
         )
 
         return bool(command and command[0] == PACKIT_HELP_COMMAND)